Imaging Ch 5 & 6

Haley Cordova
Flashcards by Haley Cordova, updated more than 1 year ago
Haley Cordova
Created by Haley Cordova about 5 years ago


Flashcards on Imaging Ch 5 & 6, created by Haley Cordova on 02/23/2016.

Resource summary

Question Answer
Three main components of an x-ray system Tube Operating console High-voltage generator
What material are x-ray tables made of? Carbon fiber
Ancillary equipment for tables Shoulder, foot supports Hand grips Compression bands
Most flexible type of tube support Ceiling mounted
Amount of Hz coming into incoming line current 60 Hz
Amount of voltage coming into incoming line current 220-240 volts
Amperage in filament current 3-8 amps
3 main conditions for x-ray production High number of electrons High voltage/potential difference Direct current
Two sections of x-ray circuit Main circuit Filament circuit
What two sections does the main circuit contain? Primary/low voltage side Secondary/high voltage side
What two features are in the filament circuit? Filament Rheostat
What does the filament do? Provides a high number of electrons
Rheostat Variable resistor - adjusts amperage by varying resistance
What components are located on the control console? kVp mA time rotor switch exposure switch
Exposure switch is designed to prevent exposure before The anode is up to speed
What does the filament circuit do? Modifies current to filament in tube Heats filament to boil off electrons Controls mA of tube current
6 components of filament circuit (in order) Incoming power Circuit breaker/main switch Line monitor and compensator Rheostat Focal spot selector Filament transformer
What do the line monitor and compensator do? Adjust for fluctuations in incoming power Tube needs exactly 220 or 240 volts
Focal spot selector Energizes small or large filament which determines small or large focal spot
Filament transformer - step up or step down? Step down
Filament transformer increases _______ and decreases _________ Increases amperage Decreases voltage
Components of main circuit - primary side Incoming power Circuit breaker, main switch Line compensator and monitor Autotransformer (kVp selector) kVp meter Timer
Is autotransformer step-up or step-down? Can be either, it's a variable transformer
Turns ratio for filament transformer Always <1
Turns ratio for autotransformer Variable, can be <1 or >1
Does the kVp meter register kVp before or after exposure? Before
kVp meter displays Voltage at secondary side of autotransformer Displays kilovolts as a result of step-up transformer
What does the timer do? Controls duration of exposure
Where is timer located in main circuit? Between autotransformer and high-voltage transformer
What is the first component of the secondary/high-voltage side of the main circuit? High-voltage transformer
Turns ratio of high-voltage transformer Fixed ratio of either 500:1 or 1000:1
When you select kVp, what two devices transform the incoming voltage into the kVp you selected? kVp selector/autotransformer High-voltage transformer
Where is the mA meter located in the main circuit? Between high-voltage transformer and x-ray tube
AEC registers mAs before or after exposure? After
Rectifier Change AC to DC before it reaches tube
Where is rectifier located? Between high-voltage transformer and x-ray tube
Transformer needs __ current; tube needs __ current transformer - AC tube - DC (this is why rectifier is located between transformer and tube)
X-ray generator is a term for all components of the circuit that: Provide electrical power to the x-ray tube
5 types of generators Single-phase; 2 pulse Three-phase; 6 pulse or 12 pulse High frequency power generator Falling load generator Mobile unit generators
Single-phase generator - pulses per second 120 pulses per second
Average voltage produced by single-phase generator 70% of kVp
Voltage ripple of single-phase generator 100% (drops all the way to zero)
Dead zone Low or no power Only occurs in single-phase Occurs 120 times per second
Three-phase power, 6 pulse 3 separate sine waves generated in 3 separate phases
Pulses per second in three-phase, 6 pulse 360 pulses per second
Voltage ripple in three-phase, 6 pulse 13 - 25%
Three-phase, 6 pulse produces _____% more x-ray photons than single-phase 35%
Pulses per second in three-phase, 12 pulse 720 pulses per second
Three phase, 12 pulse produces ___% more x-ray photons than single-phase 41%
Voltage ripple in three-phase, 12 pulse 4-10%
Advantages of 3Φ over 1Φ Shorter exposure time Higher average kVp Greater x-ray photon energy Greater x-ray photon production efficiency
Disadvantage of 3Φ compared to 1Φ More costly
Pulses per second in high-frequency generator Up to 12,000 pulses per second
Voltage ripple in high-voltage generatror 3-4% or >1?? Very small
High-voltage generators produce ___% more energy than single-phase 45%
Advantages/disadvantages of high-frequency generators Advantages: Small More efficient; almost constant kVp Increased radiation quality/quantity Disadvantage: expensive
Less voltage ripple = Higher x-ray quality and quantity
Single-phase tube generation factor (energy) 1.0
3Φ6P; 3Φ12P; and High-frequency tube generation factor (energy) 1.4
Falling load generator used with mAs timers mA falls during exposure automatically uses highest possible mA and shortest time
Advantages of falling load generator Guaranteed shortest exposure time Simpler, use with AEC
Disadvantages of falling load generator Shortens tube life due to filament wear
2 types of mobile x-ray generators Capacitor-discharge Battery powered
Capacitor Device that accumulates and stores an electrical charge
Insulator material inside capacitors Dielectric
Capacitor-discharge generator Discharges all power completely during one exposure
Advantages of capacitor-discharge generator Smaller
Disadvantages of capacitor-discharge generator Voltage falls throughout exposure Limited mAs Long exposure times Must be charged after every exposure
How does a capacitor store energy? When connected to DC current, electrons flow into the capacitor and are collected and stored on the negative place
What rate does voltage fall at in a capacitor-discharge unit 1kV/mAs Ex: 20mAs @ 70kV kV would drop by 20 (70 to 50kV) by end of exposure
Battery-powered generator Generator powered by bank of batteries that is charged from AC wall outlet
What are batteries in battery-powered generator made of? Nickel-cadmium
Advantages/disadvantages of battery-powered generators Advantages: Constant voltage & mAs during exposure Disadvantages: Larger and heavier than capacitor-discharge units
Power rating equation for three-phase and high frequency kW = (mA x kVp)/1000
Power rating equation for single phase kW = (mA x kVp x 0.7)/1000 [mult. by 0.7 because of dead zone]
Characteristics of synchronous timers in sync with 60 Hz generator must be reset after each exposure minimum exposure time is 1/60s
Electronic timer Most common and most accurate Wide range of settings, as small as 1ms
mAs timer Terminates when desired mAs is reached Automatically provides shortest exposure time at largest mA
Automatic exposure control AEC/AED/AECD Measures radiation at the IR and terminates exposure when AEC receives preset amount of radiation
2 types of AEC Phototimer Ionization chambers
Phototimer Located under IR Uses photomultiplier Light sensitive
Ionization chambers Located between patient & IR Radiation sensitive
Minimum reaction or response time 1ms Minimum time needed for AEC to respond to radiation determined by manufacturer
AEC/AED Back up time Manual exposure time set in case AEC fails Set at 1.5-2 times the expected mAs
What does the x-ray tube consist of? Anode and cathode enclosed in a vacuum glass envelope and protective housing
What is required in order to produce xrays? Source of electrons (filament) Target material (anode) High voltage Vacuum tube
Parts of cathode Filament Focusing cup Grid-biased tube
Functions of cathode Produce thermionic cloud Focus electron stream Conduct voltage between anode & cathode
What is the filament? Small coil of thorium tungsten wire that produces an electron cloud
What temperature does thermionic emission occur at? 2200°C
Melting point of tungsten 3370°C
Focusing cup Shallow nickel depression housing filaments
What does the focusing cup do? Narrows the thermionic cloud Focuses electrons Has a negative charge
Space-charge effect Point at which no more electrons can be boiled off the filament
Saturation current Point at which all available electrons in electron cloud have been sent across the tube
Grid biased tube A third wire adds a low current, positive charge to the focusing cup
What does the filament current do? Creates electron cloud or space charge
What does the tube current do? Draws electrons to anode
Parts of the anode Focal spot Stator Rotor
What is a stationary anode made of? What is the target angle? Where are they used? Tungsten in copper 45° Used in dental units & low mA machines
Other terms used for target Focus, focal point, focal spot, focal track
Why is tungsten used in the target? High melting point, dissipates heat well
How fast do rotors spin? 3600rpm High speed rotors can spin up to 10,000rpm
What is the rotating anode disk made of? Molybdenum, backed with graphite
What is the target made of? Tungsten with rhenium
What is the shaft/neck made of? Molybdenum
What is the rotor made of? Copper
Line focus principle Used to reduce effective focal spot and control mA As angle of target decreases, focal spot size and field size decrease
What is the focal spot size determined by? Filament size/length Larger filament creates large focal spot; smaller filament creates small focal spot
What degree are most targets angled at? 12°
Advantages/disadvantages of large focal spot Advantages: allows use of higher exposures, allows greater amount of heat Disadvantages: Decreased spatial resolution (decreased image quality)
Advantages/disadvantages of small focal spot Advantages: Increased spatial resolution Disadvantages: Limited to lower mA due to concentration of heat in a smaller area
Anode heel effect Radiation intensity is greatest towards cathode side of tube
Cathode end produces ___% more photons, while anode end produces ____% less Cathode - 20% more photons Anode - 25% less photons
What can decrease the anode heel effect? Increased anode angle (focal spot)
How would a smaller focal size effect anode heel effect? Smaller focal size would increase anode heel effect
How does SID affect the anode heel effect? Increased SID decreases anode heel effect Decreased SID increases anode heel effect
What is the stator composed of? Induction motor Electromagnets
Where is the stator located? OUTSIDE the glass housing
What is the rotor made of and where is it located? Copper and iron Located inside tube
What are the two stages that occur when you press the exposure button? First stage - rotor accelerates, filament heats to thermionic emission Second stage - voltage applied to tube, electrons accelerated and slammed into anode, x-rays photons produced
Envelope Pyrex glass that encloses cathode and anode (NOT stator) Creates vacuum for electrons to flow with little friction Contains a window or thinner area for electrons to exit the tube
Function of protective housing Control radiation leakage, protect against electrical shock, dissipate heat
What is protective housing made of? Lead, filled with dielectric oil
Useful beam X-rays emitted from window of tube that are used to produce the image
Leakage radiation X-rays that escape the housing
Off-focus radiation Extra focal radiation not produced at the focal spot
Heat unit equation HU = mAs x kVp x tube generator factor x number of exposures
Causes of tube failure Anode cracking Ball bearing heat damage in rotor Tube arcing Filament breakage due to vaporization
What causes anode cracking? Overheated or near max exposure on a cold anode
What causes ball bearing damage in the rotor? Overheated ball bearings warp and cause rotor to rotate improperly (wobbly rotor)
What causes tube arcing? Tungsten vaporization - tungsten evaporates and deposits on the side of the tube
What is a gassy tube? Evaporated tungsten on the inside of a tube - gassy tubes are no longer vacuums
What is anode pitting? Anode focal track becomes roughened due to vaporization
What is anode melting? Induction motor fails and rotor stops rotating Entire exposure now hits one spot on the anode and melts it
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